People can sense events and perceive the world thanks to their perceptual systems, which are interconnected parts of the body that deliver sensory and perceptual information (Gibson, 1966; Withagen & Michaels, 2005). As you will learn in this chapter, humans have six perceptual systems:

We will present each perceptual system in two steps. First, we examine the psychology of the system: the information it delivers and the psychological principles through which it works. Next, we’ll explore the system’s biology: the neural mechanisms that enable it to do its job. We will, in other words, focus first at person and mind levels of analysis (see Chapter 1) and then at the level of the nervous system and brain.

Next in the chapter, we examine three phenomena in which social factors influence sensory and perceptual experience: (1) the experience of pain; (2) the perception of faces; and (3) motivated perception, that is, the influence of motivational states on perceptual experience. We conclude with selective attention, in which you “zoom in” on streams of information in the environment, focusing on one while avoiding distraction from others.

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Before we begin, here is a word on defining sensation and perception. One approach to defining them refers to biological processes. Sensation occurs when cells at the periphery of the body detect physical stimuli; perception occurs when systems in the brain process these signals and produce conscious awareness of sensory inputs. These definitions accurately portray steps of biological processing, but present two problems when one turns to psychological experiences. First, sensory experiences result from activity in the brain, not merely in the periphery of the body; Chapter 3 explains that activating a brain region known as the sensory cortex causes people to experience bodily sensations. Second, perception can occur without conscious awareness. In studies of unconscious perception (also called subliminal perception), researchers present information so briefly that participants are not aware of its content. Nonetheless, the participants are affected by the information, which means they must have perceived it (see, e.g., Marcel, 1983; Siegel et al., 2013).

A different approach to defining sensation and perception is psychological. Two everyday examples illustrate the difference in psychological experiences that this approach captures: (1) If you bump your knee, say, “Ow, it hurts!” and someone asks, “Where does it hurt?” you point to your knee. (2) If you see a traffic accident, say, “Wow, an accident!” and someone says, “Where do you see it?” you do not point to your eye. Case #1 is an example of sensation, a feeling that is located at a point in the body, whereas #2 is an example of perception, a psychological process in which people acquire information about objects and events in the environment (Hacker, 2004, 2010).

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